Use default serde impls for Entity (bevyengine#6194)

# Objective

Currently for entities we serialize only `id`. But this is not very expected behavior. For example, in networking, when the server sends its state, it contains entities and components. On the client, I create new objects and map them (using `EntityMap`) to those received from the server (to know which one matches which). And if `generation` field is missing, this mapping can be broken. Example:

1. Server sends an entity `Entity{ id: 2, generation: 1}` with components.
2. Client puts the received entity in a map and create a new entity that maps to this received entity. The new entity have different `id` and `generation`. Let's call it `Entity{ id: 12, generation: 4}`.
3. Client sends a command for `Entity{ id: 12, generation: 4}`. To do so, it maps local entity to the one from server. But `generation` field is 0 because it was omitted for serialization on the server. So it maps to `Entity{ id: 2, generation: 0}`.
4. Server receives `Entity{ id: 2, generation: 0}` which is invalid.

In my game I worked around it by [writing custom serialization](https://github.com/dollisgame/dollis/blob/master/src/core/network/entity_serde.rs) and using `serde(with = "...")`. But it feels like a bad default to me.

Using `Entity` over a custom `NetworkId` also have the following advantages:

1. Re-use `MapEntities` trait to map `Entity`s in replicated components.
2. Instead of server `Entity <-> NetworkId ` and `Entity <-> NetworkId`, we map entities only on client.
3. No need to handling uniqueness. It's a rare case, but makes things simpler. For example, I don't need to query for a resource to create an unique ID.

Closes bevyengine#6143.

## Solution

Use default serde impls. If anyone want to avoid wasting memory on `generation`, they can create a new type that holds `u32`. This is what Bevy do for [DynamicEntity](https://docs.rs/bevy/latest/bevy/scene/struct.DynamicEntity.html) to serialize scenes. And I don't see any use case to serialize an entity id expect this one.

---

## Changelog

### Changed

- Entity now serializes / deserializes `generation` field.

## Migration Guide

- Entity now fully serialized. If you want to serialze only `id`, as it was before, you can create a new type that wraps `u32`.

crates/bevy_ecs/Cargo.toml

@@ -26,7 +26,7 @@ thread_local = "1.1.4"
 fixedbitset = "0.4"
 fxhash = "0.2"
 downcast-rs = "1.2"
-serde = "1"
+serde = { version = "1", features = ["derive"] }
 
 [dev-dependencies]
 rand = "0.8"

crates/bevy_ecs/src/entity/mod.rs

@@ -31,12 +31,11 @@
 //! [`EntityMut::insert`]: crate::world::EntityMut::insert
 //! [`EntityMut::remove`]: crate::world::EntityMut::remove
 mod map_entities;
-mod serde;
 
-pub use self::serde::*;
 pub use map_entities::*;
 
 use crate::{archetype::ArchetypeId, storage::SparseSetIndex};
+use serde::{Deserialize, Serialize};
 use std::{convert::TryFrom, fmt, mem, sync::atomic::Ordering};
 
 #[cfg(target_has_atomic = "64")]
@@ -104,7 +103,7 @@ type IdCursor = isize;
 /// [`EntityMut::id`]: crate::world::EntityMut::id
 /// [`EntityCommands`]: crate::system::EntityCommands
 /// [`Query::get`]: crate::system::Query::get
-#[derive(Clone, Copy, Hash, Eq, Ord, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Deserialize, Eq, Hash, Ord, PartialEq, PartialOrd, Serialize)]
 pub struct Entity {
     pub(crate) generation: u32,
     pub(crate) id: u32,